In every mobile radio receiver, the transmission and reception clocks must be synchronized before a data link is set up to one or more base stations. This is generally achieved by means of a three-stage method, in which the mobile radio receiver is synchronized to the slot and frame boundaries of the corresponding base station, and identifies the scrambling codes used. In accordance with the 3rd generation mobile radio system standards, such as 3GPP TS 25.211 V4.3.0 (2001-12), base stations can in general be operated in a so-called “transmission diversity mode” (also referred to as a “TX diversity mode”). In this case, the transmission signal is emitted from two different antennas and, by way of example, is modulated on one of the two antennas with a specific signal sequence, so that the two transmission signal streams are transmitted orthogonally with respect to one another in time.
Transmission diversity methods can significantly improve the performance by appropriate demodulation when the data is received at the mobile terminal. For this purpose, it is, however, necessary for the terminal to know that a transmission diversity method is being used and what transmission diversity method is being used, since, otherwise, this would result in additional performance degradation. In consequence, it is desirable that the transmission diversity mode be detected as reliably as possible at as early a time as possible, in order to ensure efficient data reception.
In principle, three different approaches are known from the prior art for solving the detection problem.                A) By means of layer 3 signaling (in this context, see the paper “An alternative scheme to detect the STTD encoding of PCCPCH” by Texas Instruments in TSG-RAN WG1 meeting #3, Nynasham (Sweden), 22 to 26 Mar. 1999, page 150 et seq.),        B) Detection of an indicator sequence which is modulated onto the synchronization channel on a symbol basis (in this context, see the paper “Fast reliable detection of STTD encoding of PCCPCH with no L3 messaging overhead” by Texas Instruments in TSG-RAN WG1 meeting #4, Yokohama (Japan), 18 to 20 Apr. 1999, page 372 et seq.),        C) Blind detection of the second transmission antenna by means of pilot sequences for example CPICH (CPICH=Common Pilot Channel) (in this context, see the paper “STTD encoding for PCCPCH” by Texas Instruments in TSG-RAN Working Group 1 meeting #2, Yokohama, 22 to 25 Feb. 1999, page 83 et seq.).        
In the methods according to A) and B) frequency synchronization by means of AFC and knowledge about the transmission channel (carrying out a channel estimation process based, for example, on the assumption that the transmission diversity mode is being used) is a precondition for obtaining any detection results at all, or at least acceptable detection results. In particular, this involves additional processing time, which is in general at the expense of the overall performance of the synchronization procedure.
The approaches according to A) and B) can thus not be used on a general basis.
In principle, methods according to C) can be carried out by means of incoherent detection methods and, within a specific frame, thus do not require frequency synchronization and/or channel information, either. However, a considerable performance degradation is observed for frequency errors of more than 1 kHz with known methods. With a frequency error of about 1.9 ppm (approximately 4 kHz), conventional approaches based on C) would detect a second transmission antenna although the transmission process is being carried out via only one transmission antenna. The diagram illustrated in FIG. 2 shows the degradation behavior of these approaches as a function of the frequency error. This has been based on the transmission diversity modulation method (which is used in the UMTS system) for the CPICH signal (STTD—Space Time Transmit Diversity) with the sequences described on pages 24 to 26 of the 3GPP TS 25.211 V4.3.0 (2001-12) Standard.
In this case, the curve “Antenna(n,m)(xppm)” in the upper diagram in FIG. 2 shows the energy component of the antenna n in the decision function for the antenna m for a frequency error of x ppm.
In order to make the method of operation and advantages of the method according to the invention (which will be described further below) clear, the method procedure for conventional transmission detection approaches based on C) will be described first of all in the following text, emphasizing the disadvantages which result from it. This is because the present invention builds on the idea on which C) is based, but extending this in such a way that a “transmission diversity mode” detection apparatus designed on the basis of this method is very robust with respect to frequency errors and channel phases.
The detection method in this case uses the characteristics of the CPICH signal, which differ depending on the transmission mode being used (transmission diversity on/off), see 3GPP TS 25.211 V4.3.0 (2001-12), for examples pages 24 to 26 of this document. When the transmission diversity mode is switched on, a modulation sequence is in each case “applied” in symbols to the transmission signal at the antenna 1 and to the transmission signal at the antenna 2, with these sequences being orthogonal with respect to one another and having a minimum length of two symbols. In this context, FIG. 3 shows an STTD sequence for the CPICH.
First of all, a list will be provided at this point of the symbols and variables which are used frequently in the following text:    r(n) received complex data sample relating to the sampling time n,    rx(a)(k) received and STTD-demodulated complex data sample for the x-th received data tuple relating to the sampling time k for the antenna a,    {circumflex over (r)}x(a) received STTD-demodulated and phase-corrected complex data sample for the x-th received data tuple for the antenna a,    sa(n) complex data sample, transmitted via the antenna a, relating to the sampling time n,    An(a) n-th sample of the STTD-modulation sequence for the antenna a,    yx decision variable relating to the x-th received data tuple,    y(a) decision variable relating to the antenna a,    arg b phase angle of the complex number b,    sig{.} mathematical sign function, and    σ2 symbol energy on the assumption that the channel has the transfer function unity for both antennas.
On the simplified assumption that the channel has the transfer function unity for both antennas and that the frequency error of the terminal crystal is negligible, the received signal at symbol level is given by:r(n)=An(1)s1(n)+An(2)s2(n)+n0(n)  equation 1
Both the symbols s(n)=s1(n)=s2(n) and the sequences An(1) and An(2) are generally known at the receiver end. Coherent processing of symbol tuples {r(k);r(k+1)} where k=2n must be carried out at the receiver end in order to detect the two antenna signals. Thus:
Signal from antenna 1:
                                          r                          (              1              )                                ⁡                      (            k            )                          =                ⁢                                                            r                ⁡                                  (                  k                  )                                            ·                              A                k                                  (                  1                  )                                                      ⁢                                          s                *                            ⁡                              (                k                )                                              +                                                    r                ⁡                                  (                                      k                    +                    1                                    )                                            ·                              A                                  k                  +                  1                                                  (                  1                  )                                                      ⁢                                          s                *                            ⁡                              (                                  k                  +                  1                                )                                              +                                    n              1                        ⁡                          (              k              )                                                              =                ⁢                                            (                                                                    A                    k                                          (                      1                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      1                                        2                                                  +                                                      A                    k                                          (                      2                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      2                                        2                                                              )                        ⁢                          A              k                              (                1                )                                              +                                    (                                                                    A                                          k                      +                      1                                                              (                      1                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      1                                        2                                                  +                                                      A                                          k                      +                      1                                                              (                      2                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      2                                        2                                                              )                        ⁢                          A                              k                +                1                                            (                1                )                                              +                                    n              1                        ⁡                          (              k              )                                          and, taking account of the characteristics of the sequences An(1) and An(2),r(1)(k)=2|A|2σs12+0σs22+n1(k)  Equation 2
Signal from antenna 2:
                                          r                          (              2              )                                ⁡                      (            k            )                          =                ⁢                                                            r                ⁡                                  (                  k                  )                                            ·                              A                k                                  (                  2                  )                                                      ⁢                                          s                *                            ⁡                              (                k                )                                              +                                                    r                ⁡                                  (                                      k                    +                    1                                    )                                            ·                              A                                  k                  +                  1                                                  (                  2                  )                                                      ⁢                                          s                *                            ⁡                              (                                  k                  +                  1                                )                                              +                                    n              1                        ⁡                          (              k              )                                                              =                ⁢                                            (                                                                    A                    k                                          (                      1                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      1                                        2                                                  +                                                      A                    k                                          (                      2                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      2                                        2                                                              )                        ⁢                          A              k                              (                2                )                                              +                                    (                                                                    A                                          k                      +                      1                                                              (                      1                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      1                                        2                                                  +                                                      A                                          k                      +                      1                                                              (                      2                      )                                                        ⁢                                      σ                                          s                      ⁢                                                                                          ⁢                      2                                        2                                                              )                        ⁢                          A                              k                +                1                                            (                2                )                                              +                                    n              1                        ⁡                          (              k              )                                          and, taking account of the characteristics of the sequences An(1) and An(2):r(2)(k)=0σs12+2|A|2σs22+n1(k)   Equation 3
Using this method, it is possible to detect unambiguously in the idealized conditions above whether a signal is being emitted from the respective antenna and whether the transmission diversity mode is or is not being used. In this context, the magnitude from Equation 2 and Equation 3y(1)=|r(1)(k)|=|2|A|2σs12+n1(k)| and y(2)=|r(2)(k)|=|2|A|2σs22+n1(k)|respectively is in general compared with a well-defined threshold value TH. The decision rule is, for example, as follows:
a) y1>TH & y2≦TH→transmission diversity mode is not active
b) y1>TH & y2>TH→transmission diversity mode is active
c) y1≦TH & y2≦TH→no decision is possible
Note: in the situation where no transmission diversity mode is being used, then σs22=0
If a specific frequency error is allowed, then the received image of s(k+1) has a phase shift of Δφ with respect to the received image s(k). This phase difference in each case projects an energy component from the antenna components to be masked out in Equation 2 and Equation 3 into the result function r(1)(k) or r(2)(k), respectively, and additionally attenuates the contribution of the respective antenna to be detected.
On the basis of the above decision criteria in a)-c), the component of the projection of the antenna 1 onto the decision function y(2) as well as the attenuation contribution to the component of the antenna 1 in the decision function y(1) are of interest.
If Equation 2 and Equation 3 are extended in order to take account of any existing frequency error, then this results in the following generalized equations for the decision functions y(1) and y(2):y(1)=|√{square root over (2)}√{square root over (1+cos Δφ)}|A|2σs12+√{square root over (2)}√{square root over (1−cos Δφ)}|A|2σs22+n1(k)|  Equation 4andy(2)=|√{square root over (2)}√{square root over (1−cos Δφ)}|A|2σs12+√{square root over (2)}√{square root over (1+cos Δφ)}|A|2σs22+n1(k)|.  Equation 5
For a phase angle of Δφ=π/2, it is evident from Equation 4 and Equation 5 that the projection components each have magnitudes which are equal to the antenna components to be detected. For the above decision rules, this means that (subject to the condition that c) is not satisfied) two antennas (the transmission diversity mode is being used) are always detected, irrespective of the transmission diversity mode that is being used. This situation is illustrated once again in the diagram in FIG. 2.
The statements so far make it clear that the methods which are known from the prior art for detection of the transmission diversity mode have only a restricted performance and a significant degradation of the results must be expected particularly during an initial synchronization phase where considerable frequency errors must be expected.
In order to further illustrate the prior art, FIG. 4 shows a block diagram of an apparatus for detection of the transmission diversity mode according to the prior art.